The rising demand for reliable, cost-effective, miniature and advanced sensing devices has led the sensor manufacturing industry away from traditional microelectronic fabrication processes and has made room for modern printing techniques. Standard printing processes are used in combination with electrically functional inks to deposit the relevant electronic materials on a specific substrate.
Flexible sensors developed through various printing techniques are expected to revolutionize the electronic sensor industry. Particularly, for biomedical applications, flexible sensors have led to remarkable advancements and they are increasingly being used in day-to-day lives. Their incredible ability to be stretchable and foldable has enabled them to be used for curved surfaces and to conform to complex geometries.
Medical applications
Biomedical sensors promise to provide patients and doctors with unprecedented amounts of health data. This can be used to provide real-time monitoring and diagnostics, and can also be used for a variety or routine health testing and examinations. Here are a few examples.
Tactile sensors are used to imitate a human touch and when placed in contact with an object, they sense various parameters such as pressure, force, shear, strain, vibrations and torsion. Various fabrication techniques are developed to manufacture pressure/tactile sensors and 3D printing technology is among the top choice owing to its low cost, simplicity, customization and scalability. These sensors are used for non-invasive diagnoses, artificial skin, prosthetics, minimal access surgery and biomechanical analysis among other things.
Temperature sensors are also used extensively in biomedical applications. To provide real-time temperature monitoring, flexible temperature sensors detect the change in electrical signal of the thermosensitive material in response to a change in temperature. Polydimethylsiloxane (PDMS), an excellent thermal and electrical insulating polymer, is one of the most commonly used substrates in flexible temperature sensors. However, there are numerous other substrates that can be used including polyurethane, polyimide (PI) and biodegradable fabrics like cotton and silk. Inkjet printing is widely used for the production of many temperature sensors but it is not the only technique.
Electrochemical biosensors in the form of ‘tattoo sensors’ are also expected to play a critical role in continuous health monitoring. They are fabricated by using the common printing techniques mentioned above including screen printing, gravure printing and inkjet printing. Screen printing is perhaps the most commonly used method because of its low cost, robustness and compatibility with a variety of substrates.
Technological drivers
The adoption of flexible sensors for biomedical applications has a common denominator with flexible circuits in general. They have been made possible by advances in sensor materials as well as print fabrication techniques.
Sensor materials
Substrate and electrically active ink are the two main components of a printed flexible sensor. Some typical and desirable characteristics of a substrate are its flexibility and ability to conform while maintaining thermal and chemical stability. Substrates may be required to possess additional properties depending on the application. PI, thermoplastic polyurethane (TPU), poly(ethylene terephthalate) (PET), PDMS and even paper are some of the common substrates used in the printing of flexible sensors. PDMS, in particular, is extensively used for biomedical devices including micro-fluids and sensors. It is biodegradable and non-toxic with high stretchability.
The ink used in the printing process is composed of a main electrically functional element along with binders, additives and solvents. Metals, dielectrics or semiconductors can be used as functional elements depending on the requirements. Silver, in the form of nanoparticles or nanowires, is the most commonly used functional element. For better print quality, ink viscosity, wetting characteristics, surface tension and density must be considered.
There is also a lot of research focused on developing a custom bio-ink that can be engineered based on specific requirements. For instance, conductivity is important in certain biomedical sensors while cell support is critical in scaffolds.
Printing techniques
Printing processes are broadly divided into two main categories: impact and non-impact. The impact printing method, as the name suggests, involves the transfer of ink from the patterned object to the substrate through physical contact. On the other hand, the non-impact printing method uses nozzles to transfer the ink onto the substrate without any physical contact.
Screen printing is an impact printing method that involves the physical transfer of ink onto a substrate through direct contact. The screen-printing technology can print highly flexible electronic sensors at a low cost, with negligible material waste. A large number of printed devices can be screen-printed in a short amount of time because screen printing has capabilities like roll-to-roll production and ambient processing. Another impact printing technique is Gravure printing that offers low-cost production, quick turnaround, and high-quality printing. It is a robust printing process and uses low-viscosity inks.
Flexographic printing is an indirect contact printing method that can handle a wide range of ink thicknesses while maintaining a constant resolution. It is a roll-to-roll rotational printing process with high throughput. This printing method is not popular for developing printed sensors and is still under research.
Inkjet printing is a non-impact printing method where a design is printed by spraying droplets of ink onto the substrate rather than using any physical image-bearers. Various biomedical sensors have been developed using this printing method. An interesting application under research includes a smart bandage for chronic wounds, capable of continuously sensing and delivering oxygen.
Bioprinting, while primarily focused on tissue engineering, has also found application in developing printed biosensors. Researchers have developed various sensors using this technique including tactile sensors, strain sensors, sensors capable of detecting finger motion, 3D conductive sensors and so on.
Furthermore, 3D printing techniques are also being used for the manufacturing of flexible sensors. Highly complex 3D structures can be easily fabricated by using one of the many available 3D printing technologies. Some of the most common 3D printing technologies are based on photopolymerization, sheet lamination, materials extrusion, binder jetting, direct energy deposition and power-bed fusion.
Conclusion
While many printing technologies are performing well enough for manufacturing high-quality flexible sensors, there are still many challenges to overcome. Many leading innovative manufacturing institutes like NextFlex, HI-RESPONSE and MADRAS consortiums are working with leading scientists and engineers to overcome various challenges including parameter standardization and process environment control.
Printed flexible sensors are expected to follow an exponential growth pattern and by 2023, the wearable technology market is expected to reach $302.3 million. The adoption of innovative printing techniques for the manufacturing of flexible sensors is expected to revolutionize the biomedical industry in the coming years.
